The present invention relates to a continuous process for forming an oxide film on the surfaces of a valve-metal, and to the measurement and recording of the potential capacity of such a formed foil. Such formed foil is typically employed in the manufacture of electrolytic capacitors. The word "forming" and variations thereof are used herein to mean anodizing, as is common practice in the art.
Formed foil is conventionally made by drawing at a constant speed, a bare etched metal foil strip through a liquid electrolyte solution and applying a fixed d.c. voltage between the foil and a negative electrode that is also submersed in the electrolyte solution. In this conventional process, the formation current, the ultimate oxide film thickness, and the potential capacitance of the foil all vary in response to variations in etch ratio along the foil.
Potential capacity of the foil in a given region, is defined as the value of electrical capacity per unit square area of the foil as measured, for example by a capacitance bridge, between the foil as one electrode and a liquid electrolyte in which the foil is submersed as the other electrode. This measurement is normally carried out by using a sample of the foil to be characterized, which sample has a known square area. It has not been practical to measure the potential capacity of the immersed foil in a continuous formation machine, even by stopping the process, because portions of the foil submersed therein are only partially formed.
An arduous and costly sampling method for making this measurement is performed in practice, after a complete roll of foil, perhaps hundreds of feet long, has been formed.
It is customary to form foil in widths of one foot or more. Thereafter, the foil is typically slit to widths on the order of one to five inches. Subsequently the slit rolls of formed foil are cut to lengths that correspond to a desired capacity value. Typically each such cut length of foil has stitched to it a metal tab that becomes the anode lead of the capacitor. The length of foil is then typically overlaid with porous insulating layers and a counter-electrode and wound into a cylindrical capacitor section. The section is impregnated or immersed in a liquid electrolyte which itself becomes the cathode of the capacitor. The oxide film that is formed on both faces of the foil is the dielectric for the capacitor.
The potential capacity per unit area of formed foil may vary as much as 50% within a master roll of formed foil. A master roll is usually characterized by taking two or more sample measurements of potential capacity at widely spaced intervals along the foil. After slitting, the foil is cut into equal lengths and the capacitors made therefrom typically have capacity values ranging from +0 to +50% of a nominal value. This is often a practical method since many capacitor users specify a minimum capacity value. However, most such capacitors contain a large excess of foil and thus represent excess costs.
Alternatively, the foil is slit, and rewound into small rolls each containing a length of foil typically only 50 times the nominal desired final length for use in a section. Each such small roll is characterized for capacity so that all capacitors made from that roll will likely fall within a narrower capacity tolerance. Clearly the added cost of this approach is high and a point of diminishing returns is soon reached as it is attempted to reduce the capacity range within which capacitors are manufactured. Further, the impracticality for automating these operations has placed a lower limit on the ultimate costs of foil electrolytic capacitors.
It is therefore an object of this invention to provide a low cost method for monitoring and recording the potential capacitance of a formed film.
It is another object of this invention to provide a method for monitoring and recording the potential capacitance of a formed foil, which data represents all regions along the foil.
It is a further object of this invention to provide a method for monitoring and recording the potential capacity of a formed foil during the foil forming process.
It is a further object of this invention to make practical automatic methods for accurately cutting formed foil to lengths, each corresponding to a predetermined capacity value.
It is a further object of this invention to reduce the cost of manufacturing electrolytic foil-electrode capacitors.
It is yet a further object of this invention to provide a means for making electrolytic foil-electrode capacitors to a narrow capacity tolerance.
These and other objects will become apparent from the following description.
The objects of the present invention are realized by combining novel capacitance monitoring and recording techniques with a method for forming foil that is disclosed in an application for U.S. Pat. Ser. No. 539,360, filed Jan. 8, 1975.